Silicone compositions have become widely accepted as release coatings, which are useful to provide a surface or material which is relatively nonadherent to other materials which would normally adhere closely thereto. Silicone release compositions may be used as coatings which release pressure sensitive adhesives for labels, decorative laminates, transfer tapes, etc. Silicone release coatings on paper, polyethylene, Mylar.RTM., and other such substrates are also useful to provide non-stick surfaces for food handling and industrial packaging.
Previously developed silicone release products are typically sold as dispersions of reactive high molecular weight polysiloxane gums in organic solvents. A curing or cross-linking catalyst is added to the dispersed, low-solids mixture, the mixture is applied to a substrate, and the coated substrate is passed through an oven to evaporate the carrier solvent and cure the silicones to a relatively non-adherent release surface.
The large thermal energy input required to evaporate the solvents and allow proper curing (cross-linking) at commercially viable rates often makes heat-curable systems costly or burdensome to use. High energy costs and strict environmental regulation of solvent emissions make the use of solvent-borne silicone release compositions uneconomical. While solventless or emulsion-borne silicone release compositions address the environmental problems, high oven temperatures and expensive energy usage are not eliminated.
To overcome the disadvantages of heat-curable release compositions, radiation-curable silicone release compositions have been developed. Ultraviolet (UV) radiation is one of the most widely used types of radiation because of its low cost, ease of maintenance, and low potential hazard to industrial users. Typical curing times are much shorter, and heat-sensitive materials can be safely coated and cured under UV radiation whereas thermal energy might damage the substrate.
Several UV-curable silicone systems are known: U.S. Pat. Nos. 3,816,282 (Viventi); 4,052,059 (Bokerman et al); and 4,070,526 (Colquhoun et al) describe compositions wherein .omega.-mercaptoalkyl substituted polysiloxanes react with vinyl-functional siloxanes when exposed to UV radiation in the presence of certain photosensitizers. The compositions, however, often require scarce or expensive starting materials, have unserviceably slow cure rates, or emit offensive odors which persist in the cured products.
UV-curable silicone resins with epoxy or acrylic functionality have been found recently to have the high degree of reactivity necessary to make them suitable for release applications, while avoiding the disadvantages of other UV-curable systems. Silicone release compositions such as those described in U.S. Pat. No. 4,279,717 (Eckberg et al) and commonly assigned, copending U.S. application Ser. No. 375,676 filed May 6, 1982 are especially advantageous for their rapid curing in the presence of certain onium salt or free-radical type photocatalysts.
Cured release coatings generally exhibit exceptionally low, or "premium", release, that is, very little force is required to separate most adhesives from the silicone-treated surface. However, many commercial applications demand higher (or "tighter") release from common pressure-sensitive adhesives, and additives, called "controlled release additives" (or "CRAs") have been developed which may be added to low-release compositions to raise their release.
U.S. Pat. Nos. 4,123,664 (Sandford, Jr.) and 3,527,659 (Keil), and commonly assigned, copending U.S. application Ser. No. 416,576 filed Sept. 10, 1982, describe release-increasing CRAs based on MQ and vinyl-MQ resins for use with heat-curable silicone release systems. However, there is a need for controlled release additives which can provide different ranges of release, as required, in ultraviolet radiation-curable systems, and particularly in the aforementioned epoxy-functional polysiloxane compositions.
It has now been discovered that small amounts of polyfunctional epoxy monomers function as controlled release additives in UV-curable epoxysilicone coating compositions. Moreover, the polyfunctional epoxy monomers provide quantitative release, that is, they provide different levels of release depending on the amounts of monomers present.
All of the patents and patent applications mentioned above are hereby incorporated by reference.